In flag terminology, a swallowtail is either
- a V-shaped cut in a flag that causes the flag to end in two points at the fly; or
- any flag which has this V-shaped cut.
The name comes from the forked tail that is a common feature of the swallow species of birds.
Nautical signal flag for "A"
Common in Scandinavia, this swallowtail flag contains a vertical section in the centre of the fly.
Swallowtail with tongue
Also common in the Nordic countries, the swallowtail flag contains a third tail (the "tongue") between the other two tails.
A guidon is the general name given to a small swallowtail flag. Guidons are used to represent military units and are displayed on vehicles attached to a particular unit. In some countries (such as the United States), guidons do not necessarily have a swallow tail.
The military use of the guidon originated from the flags used by cavalry units in Europe. A variant of the guidon – the hussar cut – was used by German cavalry regiments. Instead of a straight V-shaped cut, the swallowtail in a hussar cut flag was curved.